The subject invention relates to circuit card testing apparatus and to a test probe assembly for use in such apparatus. More particularly, the subject invention relates to a minimum cost circuit card test apparatus wherein miniature test probes normally protrude from the surface of the apparatus to make contact with conductive test points of a circuit card held against the probes by force. Additionally the invention relates to a minimum cost test probe which finds use in such apparatus.
Prior art circuit card testers are known wherein a circuit card is mounted on the top surface bed board of a test fixture housing. In one prior art tester, the bed board has several small holes underlying the circuit board. The housing is closed by a lower baseboard which is movable with respect to the top bed board. A vacuum is created in the enclosed cavity between the bed board and the baseboard. The vacuum works through the bed board holes to retain the circuit card on the bed board and forces the movable baseboard toward the bed board. As the movable baseboard progresses toward the bed board, appropriate test probes mounted on the baseboard rise through corresponding apertures in the bed board and are forced into contact with conductive test points on the undersurface of the circuit card.
In conjunction with such circuit card testers of the prior art, spring loaded electrical probes of the plunger type have previously been used. An example of such a contact is illustrated in U.S. Pat. No. 3,435,168. In the plunger type of contact, the contact tip is mounted on a cylindrical plunger which fits into the interior of a base tube. A spring in the interior of the tube provides a force for biasing the contact tip to a test point.
While elegant in design, prior art circuit card testers and their associated contact pins have proved inordinately expensive. This expense is aggravated by the fact that it is often necessary to scrap expensive portions of such circuit testing equipment when the circuit card which they are designed to test is replaced by a new circuit card with a different test point pattern.
A major contributor to the excessive cost of the prior art circuit card test equipment has been the electrical probes associated therewith. This cost factor has become increasingly aggravated as the number of such test probes in electrical test fixtures has increased. Today, it is not uncommon for thousands of such probes to be required to test one circuit card. When it is considered that prior art test probes typically cost a dollar or more, the nature of the problem may be appreciated.
Several factors contribute to the undesirable expensiveness of prior art test probes. Prior art miniature probes having retractable tips, particularly those of the plunger type, require a good deal of interior and exterior structural detail on a small scale, leading to expensive assembly and manufacture. In the prior art, these assembly procedures have been complicated by the necessity to assure proper electrical contact between the sliding plunger mechanism and the surrounding tube. To reduce cost, it would be desirable to reduce the number of parts and amount of metal required and to simplify the assembly procedures. At the same time, reusability of the probes must be maintained.